The present invention relates to an E/O switch controlling apparatus for controlling an E/O switch (electric field optical switch), and more particularly to an E/O switch controlling apparatus and a method for controlling the E/O switch for effectively employing the E/O switch using ferromagnetic crystal such as LiNbO.sub.3 in a good polarized state.
One of previously known electro-optical switches is an E/O switch using ferromagnetic crystal such as LiNbO.sub.3 crystal. The ferromagnetic crystal used in these E/O switches has a characteristic that refractive index changes according to an applied voltage. The E/O switch using such a characteristic has been employed.
The polarizing characteristic of the E/O switch incorporating the ferromagnetic crystal is improved when charges are accumulated in the ferromagnetic crystal. For this reason, when the E/O switch is used, a voltage for accumulating charges in the ferromagnetic crystal is applied to the ferromagnetic crystal, thereby charging the ferromagnetic crystal.
However, it is known that the polarizing characteristic of the E/O switch is abruptly deteriorated when it is used continuously used for a prescribed time or longer. This is because the charges accumulated in the ferromagnetic crystal exceeds a suitable value when charging for the ferromagnetic crystal is continued.
When the charges accumulated in the ferromagnetic crystal exceed a suitable value, the polarizing characteristic of light incident on the E/O switch is greatly deteriorated so that the insertion loss of the P-wave (parallel polarizing wave) and S-wave (vertical polarizing wave) is increased and crosstalk is increased. In this state, when the operation of the E/O switch is forcibly continued, as the case may be, "offset displacements" that an applied voltage exceeds a prescribed voltage occurred.
Conventionally, the characteristic of the E/O switch was recovered by properly discharging the excessive charges accumulated in the ferromagnetic crystal. Namely, when the E/O switch was continuously used, excess charging was prevented by making exchange between a charging operation of accumulating charges in the ferromagnetic crystal and a discharging operation of discharging the accumulated charges at regular intervals.
However, in the conventional method, alternate exchange between the charging operation and discharging operation requires a certain time while the E/O switch is not operated. This is inferior in time efficiency.
As described above, the ferromagnetic crystal, when charged, falls in a state with good polarizing characteristic. However, the characteristic thereof is greatly deteriorated while it is discharged. Therefore, it was very difficult to adapt the E/O switch to the characteristics of the ferromagnetic crystal during both its charging and discharging. Namely, the E/O switch could not used during the discharging operation.
In the conventional method, it was necessary to make exchange into the discharging operation before the ferromagnetic crystal within the E/O switch is excessively charged. For this purpose, the elapsed time during the charging operation was counted. Further, in order to use the E/O switch in a state with good polarizing characteristic, as the case may be, the discharging operation was made for a longer time than the charging operation. In this case, the using efficiency of the E/O switch was further attenuated.
In order to solve the above problem, the present invention intends to provide an E/O switch controlling apparatus which permits an E/O switch using ferromagnetic crystal to be used efficiently and continuously in a stable state.